Apparatus and method for reducing residual solvent levels

ABSTRACT

A method of reducing residual solvent levels in extracted biomass that is contaminated with HFC solvent includes sparging the biomass extract with a solvent stripping gas such as air or Nitrogen.  
     Apparatus for carrying out the method include a vessel ( 12;25 ) containing extract ( 26 ) and a source of the gas. The source may be in the form of a moveable, hollow wand ( 27 ) having a sparging gas outlet; or a fixed gas inlet ( 30 ) for bubbling the gas through the extract ( 26 ).

[0001] This invention concerns apparatuses and a method for reducingresidual solvent levels, especially following “extraction” of biomass.This is the extraction of flavours, fragrances or pharmaceuticallyactive ingredients from materials of natural origin (these materialsbeing referred to as “biomass” in the body of this text).

[0002] Examples of biomass materials include but are not limited toflavoursome or aromatic substances such as coriander, cloves, staranise, coffee, orange juice, fennel seeds, cumin, ginger and other kindsof bark, leaves, flowers, fruit, roots, rhizomes and seeds. Biomass mayalso be extracted in the form of biologically active substances such aspesticides and pharmaceutically active substances or precursors thereto,obtainable e.g. from plant material, a cell culture or a fermentationbroth.

[0003] There is growing technical and commercial interest in usingnear-critical solvents in such extraction processes. Examples of suchsolvents include liquefied carbon dioxide or, of particular interest, afamily of chlorine-free solvents based on organic hydrofluorocarbon(HFC) species.

[0004] By the term “hydrofluorocarbon” we are referring to materialswhich contain carbon, hydrogen and fluorine atoms only and which arethus chlorine-free.

[0005] Preferred hydrofluorocarbons are the hydrofluoroalkanes andparticularly the C₁₋₄ hydrofluoroalkanes. Suitable examples of C₁₋₄hydrofluoroalkanes which may be used as solvents include, inter alia,trifluoromethane (R-23), fluoromethane (R-41), difluoromethane (R-32),pentafluoroethane (R-125), 1,1,1-trifluoroethane (R-143a),1,1,2,2-tetrafluoroethane (R-134), 1,1,1,2-tetrafluoroethane (R-134a),1,1-difluoroethane (R-152a), heptrafluoropropanes and particularly1,1,1,2,3,3,-heptafluoropropane (R-227ea), 1,1,1,2,3,3-hexafluoropropane(R-236ea), 1,1,1,2,2,3-hexafluoropropane (R-236cb),1,1,1,3,3,3-hexafluoropropane (R-236fa), 1,1,1,3,3-pentafluoropropane(R-245fa), 1,1,2,2,3-pentafluoropropane (R-245ca),1,1,1,2,3-pentafluoropropane (R-245eb), 1,1,2,3,3-pentafluoropropane(R-245ea) and 1,1,1,3,3-pentafluorobutane (R-365mfc). Mixtures of two ormore hydrofluorocarbons may be used if desired.

[0006] R-134a, R-227ea, R-32, R-125, R-245ca and R-245fa are preferred.

[0007] An especially preferred hydrofluorocarbon for use in the presentinvention is 1,1,1,2-tetrafluoroethane (R-134a).

[0008] It is possible to carry out biomass extraction using othersolvents, such as chlorofluorocarbons (“CFC's”) orhydrochlorofluorcarbons (“HCFC's”) and/or mixtures of solvents.

[0009] Known extraction processes using these solvents are normallycarried out in closed-loop extraction equipment. A typical example 10 ofsuch a system is shown schematically in FIG. 1.

[0010] In this typical system, liquefied solvent is allowed to percolateby gravity in downflow through a bed of biomass held in vessel 11.Thence it flows to evaporator 12 where the volatile solvent vapour isvaporised by heat exchange with a hot fluid. The vapour from evaporator12 is then compressed by compressor 13. The compressed vapour is nextfed to a condenser 14 where it is liquefied by heat exchange with a coldfluid. The liquefied solvent is then optionally collected inintermediate storage vessel (receiver) 15 or returned directly to theextraction vessel 1 to complete the circuit.

[0011] The extraction of flavours, fragrances or pharmaceutically activecomponents from materials of natural origin using chlorine-free solventsbased on HFC's and other solvents as noted is of growing technical andcommercial interest. One reason for use of the closed loop extractioncircuit of FIG. 1 is to avoid the undesirable release of HFC or othersolvents to atmosphere.

[0012] Residual solvent levels in extracts to be used in foodstuffs aregoverned by legislation in many countries. The levels of residualsolvents in extracts for other applications are generally less strictlydefined. Even where legislation does not dictate a specific level ofsolvent residue, it is generally desirable to minimise the residuelevels where economic and practicable to do so. In the prior art,residual solvent residues have been reduced through a combination ofheating and evacuation, or in the case of CO₂ solvents by relativelymild evacuation alone. Unfortunately, the process of heating andevacuation often has a detrimental effect on the aroma of the finishedextract, many of the volatile “top note” components having been removedalong with the solvent.

[0013] One of the key benefits of using a solvent such as1,1,1,2-tetrafluoroethane (R-134a) for extraction of materials ofbiological origin is its ability to capture the relatively volatileflavour and fragrance components. In order to maintain the high qualityof the extracts obtained with R-134a, a method of reducing the residualsolvent levels that does not impact significantly on the organolepticbehaviour of the extract is required.

[0014] According to a first aspect of the invention there is providedapparatus as defined in claim 1.

[0015] According to a second aspect of the invention there is provided amethod as defined in claim 5.

[0016] Preferred features of the apparatus and method are set out in thedependent claims.

[0017] There now follows a description of preferred embodiments of theinvention, by way of non-limiting example, with reference being made tothe accompanying drawings in which:

[0018]FIG. 1 is a schematic representation of a prior art biomassextraction plant;

[0019]FIG. 2 shows part of the FIG. 1 plant modified to include featuresin accordance with the invention;

[0020]FIG. 3 shows an alternative vessel, according to the invention, inwhich the invention may be practised; and

[0021]FIGS. 4 and 5 are art graphs illustrating the results ofcomparative experiments including the method of the invention.

[0022]FIG. 2 shows part of the FIG. 1 circuit, modified in accordancewith the invention. In FIG. 2 components having the same function astheir FIG. 1 counterparts have like reference numerals and are notdescribed again in detail.

[0023] The solvent vapour delivery line 12 a exiting the upper part ofthe evaporator 12 is switchably connectable, by means of flow controlvalve 29, to compressor 13 (when the apparatus is in use to extractbiomass); to a vacuum (for purging evaporator 12 at the end of theextraction process) or to a sparging gas disposal line 31.

[0024] Flow control valve 29 is controllable e.g. manually or by meansof a computer.

[0025]FIG. 2 shows a sparging wand 27 that is immersible into thebiomass extract liquor 26 in evaporator 12 that, during and immediatelyafter extraction, is contaminated with HFC solvent.

[0026] Wand 27 is a hollow elongate member having one or more aperturestherein. Wand 27 is open at one end that is connected to a supply line27 a of pressurised sparging gas such as air or nitrogen.

[0027] Wand 27 may be inserted into evaporator 27 e.g. via an accessplate or hatch and submerged under the surface of the extract 26. Whensparging gas flows through line 27 a and into the hollow interior 27 itexits via the apertures in wand 27 and perfuses through liquor 26.

[0028] An alternative arrangement is shown in FIG. 3, in which thesparging occurs in a hollow vessel 25 remote from evaporator 12. In thiscase a wand 27 could be used in the same way as in FIG. 2.Alternatively, as shown, a sparging gas (air or N₂) supply 30 may bepermanently connected via a port beneath the surface of liquor 26 in thevessel 30 or evaporator 12.

[0029] The proposed method of residual HFC solvent reduction is asfollows. After the extraction process is complete, the collected extractis subject to mild evacuation, e.g. by means of the vacuum connection inFIG. 2, to remove the bulk of the solvent for recovery. At this stagethe residual solvent level is likely to be of the order of 10000 ppm.The sparging gas is then introduced into the extract e.g. using the wand27 and sparging is continued at a rate and period sufficient to achievethe desired level of solvent residue, typically of the order of 1-100ppm.

[0030] During sparging the evaporator or vessel outlet may be connectedto sparging gas disposal line 31.

[0031] This process may be conducted within the evaporator/collector 13of the extraction equipment (FIG. 2 method); in a separate vessel (FIG.3 method) or storage container 25 for the extract; or in acountercurrent stripping column where the (liquid) extract is passedover a packing material in contact with the sparging gas. While air canbe used as the sparging gas to good effect, the use of nitrogen isbeneficial in that it provides an inert atmosphere over the extract thusimproving storage stability, especially when sparging is conducted inthe extract storage vessel itself. When the extract is in the form of alow melting solid or paste, the extract may beneficially be warmedbefore sparging, to generate a liquid.

[0032] The efficiency of the method of the invention is illustrated bythe following non-limiting example:

EXAMPLE

[0033] The samples were subjected to a combination of heat (up to 4° C.)and vacuum for known periods of time as well as nitrogen sparging.Graphs were plotted which indicated the rate of removal of1,1,1,2-tetrafluoroethane (“R-134a”).

[0034] Removal methods used; Conditions a Before b 10 mins @ 40° C. c 10mins vacuum @ RT d 10 mins vacuum @ 40° C. e 20 mins vacuum @ 40° C. fN2 sparge 10 mins @ RT g N2 sparge 20 mins @ RT

[0035] In order to determine the amount of residual R-134a the extract(0.5 g) was weighted into a 30 mil Hypo-seal vial and crimp sealed witha PTFE coated silicone rubber septum. The vial was heated to 80° C. for30 mins to liberate the R-134a from the extract. The vial was removedfrom the oven and pressurised with 20 mls of air from a syringe. Thesyringe was then allowed to refill and immediately injected onto the GCvia a gas sample valve. The GC had been previously calibrated.

[0036] The results were then calculated in % (or ppm) w/w R-134a/weightof biomass or extract. The limit of detection was around 50 ppm w/wR-134a in the extract.

[0037] FIGS. 4 and 5 show the results of the treatments.

DISCUSSION

[0038] For the star anise extract used in this experiment the use ofvacuum and heat (40° C.) will remove the relatively large quantities ofresidual R-134a in a relatively short space of time (i.e. 2700 to 100ppm w/w in less than 20 mins) but with prolonged time required forfurther reduction. Similar results are expected from other extracts.

[0039] It can be seen from FIG. 5 that the most effective means ofremoval of R-134a is the nitrogen sparge method (10,000 to <1 ppm w/w in10 mins). This method also has the added advantage of not requiring anyheat treatment of the extract. In addition nitrogen sparging provides aninert atmosphere for further storage.

1. Apparatus for reducing residual solvent levels in extracted biomass,comprising a vessel for containing liquid biomass extract contaminatedwith solvent; and a gas supply for sparging biomass extract in thevessel with a solvent stripping vapour.
 2. Apparatus according to claim1 wherein the gas supply includes a hollow wand having a gas outlet andbeing connected to a source of pressurised solvent stripping vapour,whereby the solvent stripping vapour flows from the source through thehollow wand to exit via the gas outlet, the gas outlet of the wand beingmoveably immersed in the liquid biomass extract to permit spargingthereof by the solvent stripping vapour.
 3. Apparatus according to claim1 wherein the gas supply includes a gas inlet rigidly secured to supplysolvent stripping vapour, through a wall of the vessel, to sparge anybiomass extract in the vessel.
 4. Apparatus according to claim 1 whereinthe gas supply supplies air and/or Nitrogen for sparging of biomassextract.
 5. A method of reducing solvent levels in extracted biomass,comprising sparging liquid biomass extract, contaminated with solvent,with a solvent stripping vapour.
 6. A method according to claim 5wherein the solvent stripping vapour is air or Nitrogen.
 7. Biomassextract obtained by the method of claim 5 or claim 6.